Use one base class for plain_coap and coap

[Cyber1000]

Since plain_coap has some similarities to coap (except the AES-encryption) I now use a common base between these two. That will also reduce the changes involved moving to another coap-libary (for example aiocoap) in the near future.

Tested with coap, need someone testing this with plain_coap

Depends on #85 (including commits from there), so should be merged afterwards

• Merge stop observing #85 before
• Test if plain_coap is still working

[Cyber1000]

To just see changes from this PR, have a look at Cyber1000/py-air-control@master...Cyber1000:coap
(sorry github doesn't provide dependant PRs)

[rgerganov]

Thanks, I have merged #85, please rebase.

[Cyber1000]

rebased and tested (with coap)

[rgerganov]

Thanks! I wish someone can confirm if this works with plain coap before merging.

[Cyber1000]

Hoping this too, I also have added a todo, cause I found something I didn't need in coap and don't think I'll need it in plain_coap, but can't verify it myself.
I'll go on with my changes (in further PRs), hoping that meanwhile someone can test this.

[rgerganov]

@shexbeer Could you please test this PR with your device? We want to make sure this change won't break the support for plain_coap. Thanks!